Small perf optimizations preparing sampled images in vulkan

In our current setup, there is no need for storing the sampled images in an
array and then putting in barriers for them later. If we ever change the
system to building up these secondary command buffers early, we will need
to go back to storing the sampled images.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2302333002

Review-Url: https://codereview.chromium.org/2302333002
2 files changed